Imagine a world without bees, butterflies, and flowering plants. That was Earth 125 million years ago.

Turn back the clock 400 million years, and there were no trees.

At 450 million years in the past, even the earliest insects had not yet developed.

And looking back 500 million years-a half-billion years before the present-the land was devoid of life, which at that time flourished in a profusion of strange forms in the oceans.

These and other major turning points are the amazing story of evolution, the most remarkable force in the history of Earth, the organizing principle throughout the biological sciences, and the most important mechanism scientists use to understand the varieties of life on our planet.

To learn about these major transitions, each of which brought forth new possibilities for life, is to embark on an unforgettable look into the past. It’s also a captivating opportunity to get a deeper understanding of how evolution works, to draw intricate connections between living things, and to think about life-not just yours but the lives of everything around you-in new ways.

Major Transitions in Evolution tells this science-detective story in 24 lavishly illustrated lectures that focus on the giant leaps that gave rise to nature’s boundless diversity. In a course of breathtaking scope, you study the conditions that led to the first complex cells, flying insects, flowering plants, mammals, modern humans, and many other breakthroughs. And in the process of studying the past, you gain a powerful understanding of the present world.

Given the broad scope of the subject, this course is taught by two professors: Anthony Martin, a paleontologist and geologist at Emory University, and John Hawks, a paleoanthropologist at the University of Wisconsin-Madison. Each is an outstanding teacher in his field, adept at making the subject interesting and accessible no matter what your background in science. And in the final lecture, the two appear together for an absorbing conversation on common themes in the epic saga of life on Earth.

Giant Leaps that Brought Us to Today

Among the major transitions you cover are these:

From simple to complex cells: Life’s first major evolutionary transition was the leap from basic prokaryotic to more complex eukaryotic cells, which contain a nucleus and other specialized structures. This was the crucial step that eventually led to plants and animals.

From fish to four legs: The iconic image of evolution is a fish emerging onto land. This transition might not have happened without shade provided by the newly developing forests, whose protective canopy gave the first fishapods protection from the sun.

Dinosaurs become birds: Dinosaurs didn’t go completely extinct; they survive today as birds, whose distinctive wings, feathers, and other features are visible in transitional fossils such as Archaeopteryx, from about 150 million years ago.

Modern humans: The evolution of tree-dwelling primates to upright-walking apes later led to the evolution of modern humans-a species that invented agriculture, poetry, computers, and the techniques to trace its own lineage and that of all life.

You also explore many other transitions that occurred between these milestones, and you take an intriguing look ahead to speculate about the future direction of evolution. From the deep past until today, evolution has been a story with countless subplots, false leads, and reversals of fortune. But it has had one overarching theme-that life is wondrous, resilient, and endlessly surprising.